A Phytophthora sojae effector PsCRN63 forms homo-/hetero-dimers to suppress plant immunity via an inverted association manner

Identification of Phytophthora cinnamomi CRN effectors and their roles in manipulating cell death during Persea americana infection

The oomycete Phytophthora cinnamomi is a devastating plant pathogen with a notably broad host range. It is the causal agent of Phytophthora root rot (PRR), arguably the most economically important yield-limiting disease in Persea americana (avocado). Despite this, our understanding of the mechanisms P. cinnamomi employs to infect and successfully colonize avocado remains limited, particularly regarding the pathogen's ability to maintain its biotrophic and necrotrophic lifestyles during infection. The pathogen utilises a large repertoire of effector proteins which function in facilitating and establishing disease in susceptible host plants. Crinkling and necrosis effectors (CRN/Crinklers) are suspected to manipulate cell death to aid in maintenance of the pathogens biotrophic and necrotrophic lifestyles during different stages of infection. The current study identified 25 P. cinnamomi CRN effectors from the GKB4 genome using an HMM profile and assigned putative function to them as either cell death inducers or suppressors. Function was assigned to 10 PcinCRNs by analysing their RNA-seq expression profiles, relatedness to other functionally characterised Phytophthora CRNs and tertiary protein predictions. The full-length coding sequences for these PcinCRNs were confirmed by Sanger sequencing, six of which were found to have two divergent alleles. The presence of alleles indicates that the proteins encoded may perform contradicting functions in cell death manipulation, or function in different host plant species. Overall, this study provides a foundation for future research on P. cinnamomi infection and cell death manipulation mechanisms.

The PTI-suppressing Avr2 effector from Fusarium oxysporum suppresses mono-ubiquitination and plasma membrane dissociation of BIK1

Plant pathogens use effector proteins to target host processes involved in pathogen perception, immune signalling, or defence outputs. Unlike foliar pathogens, it is poorly understood how root-invading pathogens suppress immunity. The Avr2 effector from the tomato root- and xylem-colonizing pathogen Fusarium oxysporum suppresses immune signalling induced by various pathogen-associated molecular patterns (PAMPs). It is unknown how Avr2 targets the immune system. Transgenic AVR2 Arabidopsis thaliana phenocopies mutants in which the pattern recognition receptor (PRR) co-receptor BRI1-ASSOCIATED RECEPTOR KINASE (BAK1) or its downstream signalling kinase BOTRYTIS-INDUCED KINASE 1 (BIK1) are knocked out. We therefore tested whether these kinases are Avr2 targets. Flg22-induced complex formation of the PRR FLAGELLIN SENSITIVE 2 and BAK1 occurred in the presence and absence of Avr2, indicating that Avr2 does not affect BAK1 function or PRR complex formation. Bimolecular fluorescence complementation assays showed that Avr2 and BIK1 co-localize in planta. Although Avr2 did not affect flg22-induced BIK1 phosphorylation, mono-ubiquitination was compromised. Furthermore, Avr2 affected BIK1 abundance and shifted its localization from nucleocytoplasmic to the cell periphery/plasma membrane. Together, these data imply that Avr2 may retain BIK1 at the plasma membrane, thereby suppressing its ability to activate immune signalling. Because mono-ubiquitination of BIK1 is required for its internalization, interference with this process by Avr2 could provide a mechanistic explanation for the compromised BIK1 mobility upon flg22 treatment. The identification of BIK1 as an effector target of a root-invading vascular pathogen identifies this kinase as a conserved signalling component for both root and shoot immunity.

Fungal Effectoromics: A World in Constant Evolution

Effectors are small, secreted molecules that mediate the establishment of interactions in nature. While some concepts of effector biology have stood the test of time, this area of study is ever-evolving as new effectors and associated characteristics are being revealed. In the present review, the different characteristics that underly effector classifications are discussed, contrasting past and present knowledge regarding these molecules to foster a more comprehensive understanding of effectors for the reader. Research gaps in effector identification and perspectives for effector application in plant disease management are also presented, with a focus on fungal effectors in the plant-microbe interaction and interactions beyond the plant host. In summary, the review provides an amenable yet thorough introduction to fungal effector biology, presenting noteworthy examples of effectors and effector studies that have shaped our present understanding of the field.

A nematode effector inhibits plant immunity by preventing cytosolic free Ca2+ rise

The Meloidogyne enterolobii effector MeTCTP is a member of the translationally controlled tumour protein (TCTP) family, involved in M. enterolobii parasitism. In this study, we found that MeTCTP forms homodimers and, in this form, binds calcium ions (Ca²⁺ ). At the same time, Ca²⁺ could induce homodimerization of MeTCTP. We further identified that MeTCTP inhibits the increase of cytosolic free Ca²⁺ concentration ([Ca²⁺ ]_cyt ) in plant cells and suppresses plant immune responses. This includes suppression of reactive oxygen species burst and cell necrosis, further promoting M. enterolobii parasitism. Our results have elucidated that the effector MeTCTP can directly target Ca²⁺ by its homodimeric form and prevent [Ca²⁺ ]_cyt rise in plant roots, revealing a novel mechanism utilized by plant pathogens to suppress plant immunity.

The heterologous expression of conserved Glycine max (soybean) mitogen activated protein kinase 3 (MAPK3) paralogs suppresses Meloidogyne incognita parasitism in Gossypium hirsutum (upland cotton)

Two conserved Glycine max (soybean) mitogen activated protein kinase 3 (MAPK3) paralogs function in defense to the parasitic soybean cyst nematode Heterodera glycines. Gene Ontology analyses of RNA seq data obtained from MAPK3-1-overexpressing (OE) and MAPK3-2-OE roots compared to their control, as well as MAPK3-1-RNA interference (RNAi) and MAPK3-2-RNAi compared to their control, hierarchically orders the induced and suppressed genes, strengthening the hypothesis that their heterologous expression in Gossypium hirsutum (upland cotton) would impair parasitism by the root knot nematode (RKN) Meloidogyne incognita. MAPK3-1 expression (E) in G. hirsutum suppresses the production of M. incognita root galls, egg masses, and second stage juveniles (J2s) by 80.32%, 82.37%, and 88.21%, respectfully. Unexpectedly, egg number increases by 28.99% but J2s are inviable. MAPK3-2-E effects are identical, statistically. MAPK3-1-E and MAPK3-2-E decreases root mass 1.49-fold and 1.55-fold, respectively, as compared to the pRAP15-ccdB-E control. The reproductive factor (RF) of M. incognita for G. hirsutum roots expressing MAPK3-1-E or MAPK3-2-E decreases 60.39% and 50.46%, respectively, compared to controls. The results are consistent with upstream pathogen activated molecular pattern (PAMP) triggered immunity (PTI) and effector triggered immunity (ETI) functioning in defense to H. glycines. The experiments showcase the feasibility of employing MAPK3, through heterologous expression, to combat M. incognita parasitism, possibly overcoming impediments otherwise making G. hirsutum's defense platform deficient. MAPK homologs are identified in other important crop species for future functional analyses.

Pathogen effectors: What do they do at plasmodesmata?

Plants perceive an assortment of external cues during their life cycle, including abiotic and biotic stressors. Biotic stress from a variety of pathogens, including viruses, oomycetes, fungi, and bacteria, is considered to be a substantial factor hindering plant growth and development. To hijack the host cell's defence machinery, plant pathogens have evolved sophisticated attack strategies mediated by numerous effector proteins. Several studies have indicated that plasmodesmata (PD), symplasmic pores that facilitate cell-to-cell communication between a cell and neighbouring cells, are one of the targets of pathogen effectors. However, in contrast to plant-pathogenic viruses, reports of fungal- and bacterial-encoded effectors that localize to and exploit PD are limited. Surprisingly, a recent study of PD-associated bacterial effectors has shown that a number of bacterial effectors undergo cell-to-cell movement via PD. Here we summarize and highlight recent advances in the study of PD-associated fungal/oomycete/bacterial effectors. We also discuss how pathogen effectors interfere with host defence mechanisms in the context of PD regulation.

Unraveling Plant Cell Death during Phytophthora Infection

Oomycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, of which several hundred organisms are considered among the most devastating plant pathogens—especially members of the genus Phytophthora. Phytophthora spp. have a large repertoire of effectors that aid in eliciting a susceptible response in host plants. What is of increasing interest is the involvement of Phytophthora effectors in regulating programed cell death (PCD)—in particular, the hypersensitive response. There have been numerous functional characterization studies, which demonstrate Phytophthora effectors either inducing or suppressing host cell death, which may play a crucial role in Phytophthora’s ability to regulate their hemi-biotrophic lifestyle. Despite several advances in techniques used to identify and characterize Phytophthora effectors, knowledge is still lacking for some important species, including Phytophthora cinnamomi. This review discusses what the term PCD means and the gap in knowledge between pathogenic and developmental forms of PCD in plants. We also discuss the role cell death plays in the virulence of Phytophthora spp. and the effectors that have so far been identified as playing a role in cell death manipulation. Finally, we touch on the different techniques available to study effector functions, such as cell death induction/suppression.

A BTB/POZ domain-containing protein negatively regulates plant immunity in Nicotiana benthamiana

Plants have evolved immune systems to fight against pathogens. However, it is still largely unknown how the plant immunity is finely regulated. Here we identified a BTB/POZ domain-containing protein, namely NbBTB, which is predicted to be a member of the ubiquitin E3 ligase complex. The NbBTB expression is downregulated upon the oomycete pathogen Phytophthora parasitica infection. Overexpression of NbBTB in Nicotiana benthamiana promoted plant susceptibility to P. parasitica infection, and silencing NbBTB increased plant resistance to P. parasitica, indicating that NbBTB negatively modulates plant basal defense. Interestingly, overexpressing or silencing NbBTB did not affect plant resistance to two bacterial pathogens Ralstonia solanacearum and Pseudomonas syringae, suggesting that NbBTB is specifically involved in basal defense against oomycete pathogen. Expression of NbBTB suppressed hypersensitive response (HR) triggered by avirulence proteins from both R. sonanacearum and P. infestans, and silencing NbBTB showed the opposite effect, indicating that NbBTB negatively regulates effector-triggered immunity (ETI). Protein accumulation of avirulence effectors in NbBTB-silenced plants was significantly enhanced, suggesting that NbBTB is likely to negatively modulate ETI by affecting effector protein accumulation. Together, our results demonstrated that NbBTB is a negative regulator in both plant basal defense and ETI.

Two stripe rust effectors impair wheat resistance by suppressing import of host Fe-S protein into chloroplasts

Several effectors from phytopathogens usually target various cell organelles to interfere with plant defenses, and they generally contain sequences that direct their translocation into organelles, such as chloroplasts. In this study, we characterized a different mechanism for effectors to attack chloroplasts in wheat (Triticum aestivum). Two effectors from Puccinia striiformis f. sp. tritici (Pst), Pst_4, and Pst_5, inhibit Bax-mediated cell death and plant immune responses, such as callose deposition and reactive oxygen species (ROS) accumulation. Gene silencing of the two effectors induced significant resistance to Pst, demonstrating that both effectors function as virulence factors of Pst. Although these two effectors have low sequence similarities and lack chloroplast transit peptides, they both interact with TaISP (wheat cytochrome b6-f complex iron-sulfur subunit, a chloroplast protein encoded by nuclear gene) in the cytoplasm. Silencing of TaISP impaired wheat resistance to avirulent Pst and resulted in less accumulation of ROS. Heterogeneous expression of TaISP enhanced chloroplast-derived ROS accumulation in Nicotiana benthamiana. Co-localization in N. benthamiana and western blot assay of TaISP content in wheat chloroplasts show that both effectors suppressed TaISP from entering chloroplasts. We conclude that these biotrophic fungal effectors suppress plant defenses by disrupting the sorting of chloroplast protein, thereby limiting host ROS accumulation and promoting fungal pathogenicity.

A Phytophthora capsici RXLR effector targets and inhibits the central immune kinases to suppress plant immunity

Receptor-like cytoplasmic kinase subfamily VII (RLCK-VII) proteins are the central immune kinases in plant pattern-recognition receptor (PRR) complexes, and they orchestrate a complex array of defense responses against bacterial and fungal pathogens. However, the role of RLCK-VII in plant-oomycete pathogen interactions has not been established. Phytophthora capsici is a notorious oomycete pathogen that infects many agriculturally important vegetables. Here, we report the identification of RXLR25, an RXLR effector that is required for the virulence of P. capsici. In planta expression of RXLR25 significantly enhanced plants' susceptibility to Phytophthora pathogens. Microbial pattern-induced immune activation in Arabidopsis was severely impaired by RXLR25. We further showed that RXLR25 interacts with RLCK-VII proteins. Using nine rlck-vii high-order mutants, we observed that RLCK-VII-6 and RLCK-VII-8 members are required for resistance to P. capsici. The RLCK-VII-6 members are specifically required for Phytophthora culture filtrate (CF)-induced immune responses. RXLR25 directly targets RLCK-VII proteins such as BIK1, PBL8, and PBL17 and inhibits pattern-induced phosphorylation of RLCK-VIIs to suppress downstream immune responses. This study identified a key virulence factor for P. capsici, and the results revealed the importance of RLCK-VII proteins in plant-oomycete interactions.

Perturbations in nitric oxide homeostasis promote Arabidopsis disease susceptibility towards Phytophthora parasitica

Phytophthora species can infect hundreds of different plants, including many important crops, causing a number of agriculturally relevant diseases. A key feature of attempted pathogen infection is the rapid production of the redox active molecule nitric oxide (NO). However, the potential role(s) of NO in plant resistance against Phytophthora is relatively unexplored. Here we show that the level of NO accumulation is crucial for basal resistance in Arabidopsis against Phytophthora parasitica. Counterintuitively, both relatively low or relatively high NO accumulation leads to reduced resistance against P. parasitica. S-nitrosylation, the addition of a NO group to a protein cysteine thiol to form an S-nitrosothiol, is an important route for NO bioactivity and this process is regulated predominantly by S-nitrosoglutathione reductase 1 (GSNOR1). Loss-of-function mutations in GSNOR1 disable both salicylic acid accumulation and associated signalling, and also the production of reactive oxygen species, leading to susceptibility towards P. parasitica. Significantly, we also demonstrate that secreted proteins from P. parasitica can inhibit Arabidopsis GSNOR1 activity.

Phytophthora sojae leucine-rich repeat receptor-like kinases: diverse and essential roles in development and pathogenicity

Leucine-rich repeat receptor-like kinases (LRR-RLKs) are critical signal receptors in plant development and defense. Like plants, oomycete pathogen genomes also harbor LRR-RLKs, but their functions remain largely unknown. Here, we systematically characterize all the 24 LRR-RLK genes (PsRLKs) from Phytophthora sojae, which is a model of oomycete pathogens. Although none of them was required for vegetative growth, the specific PsRLKs are important for stress responses, zoospore production, zoospores chemotaxis, and pathogenicity. Interestingly, the Gα subunit PsGPA1 interacts with the five chemotaxis-related PsRLKs via their intracellular kinase domains, and expression of PsGPA1 gene is downregulated in the three mutants (ΔPsRLK17/22/24). Moreover, we generated the PsRLK-PsRLK interaction network of P. sojae and found that PsRLK21, together with PsRLK10 or PsRLK17, regulate virulence by direct association. Taken together, our results reveal the diverse roles of LRR-RLKs in modulating P. sojae development, interaction with soybean, and responses to diverse environmental factors.

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Systematically functional analysis of LRR-RLK family with 24 members in P. sojae

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Five chemotaxis-related PsRLKs directly interact with Gα protein PsGPA1

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PsRLKs form an interaction network in P. sojae

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The complex PsRLK21-PsRLK10/17 jointly regulates pathogenesis

A Conserved Oomycete CRN Effector Targets Tomato TCP14-2 to Enhance Virulence

Phytophthora spp. secrete vast arrays of effector molecules during infection to aid in host colonization. The crinkling and necrosis (CRN) protein family forms an extensive repertoire of candidate effectors that accumulate in the host nucleus to perturb processes required for immunity. Here, we show that CRN12_997 from Phytophthora capsici binds a TCP transcription factor, SlTCP14-2, to inhibit its immunity-associated activity against Phytophthora spp. Coimmunoprecipitation and bimolecular fluorescence complementation studies confirm a specific CRN12_997-SlTCP14-2 interaction in vivo. Coexpression of CRN12_997 specifically counteracts the TCP14-enhanced immunity phenotype, suggesting that CRN mediated perturbation of SlTCP14-2 function. We show that SlTCP14-2 associates with nuclear chromatin and that CRN12_997 diminishes SlTCP14-2 DNA binding. Collectively, our data support a model in which SlTCP14-2 associates with chromatin to enhance immunity. The interaction between CRN12_997 and SlTCP14-2 reduces DNA binding of the immune regulator. We propose that the modulation of SlTCP14-2 chromatin affinity, caused by CRN12-997, enhances susceptibility to P. capsici.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A Valsa mali Effector Protein 1 Targets Apple (Malus domestica) Pathogenesis-Related 10 Protein to Promote Virulence

To successfully colonize the plants, the pathogenic microbes secrete a mass of effector proteins which manipulate host immunity. Apple valsa canker is a destructive disease caused by the weakly parasitic fungus Valsa mali. A previous study indicated that the V. mali effector protein 1 (VmEP1) is an essential virulence factor. However, the pathogenic mechanism of VmEP1 in V. mali remains poorly understood. In this study, we found that the apple (Malus domestica) pathogenesis-related 10 proteins (MdPR10) are the virulence target of VmEP1 using a yeast two-hybrid screening. By bimolecular fluorescence (BiFC) and coimmunoprecipitation (Co-IP), we confirmed that the VmEP1 interacts with MdPR10 in vivo. Silencing of MdPR10 notably enhanced the V. mali infection, and overexpression of MdPR10 markedly reduced its infection, which corroborates its positive role in plant immunity against V. mali. Furthermore, we showed that the co-expression of VmEP1 with MdPR10 compromised the MdPR10-mediated resistance to V. mali. Taken together, our results revealed a mechanism by which a V. mali effector protein suppresses the host immune responses by interfering with the MdPR10-mediated resistance to V. mali during the infection.

A LysM Domain-Containing Protein LtLysM1 Is Important for Vegetative Growth and Pathogenesis in Woody Plant Pathogen Lasiodiplodia theobromae

Lysin motif (LysM) proteins are reported to be necessary for the virulence and immune response suppression in many herbaceous plant pathogens, while far less is documented in woody plant pathogens. In this study, we preliminarily characterized the molecular function of a LysM protein LtLysM1 in woody plant pathogen Lasiodiplodia theobromae. Transcriptional profiles revealed that LtLysM1 is highly expressed at infectious stages, especially at 36 and 48 hours post inoculation. Amino acid sequence analyses revealed that LtLysM1 was a putative glycoprotein with 10 predicted N-glycosylation sites and one LysM domain. Pathogenicity tests showed that overexpressed transformants of LtLysM1 displayed increased virulence on grapevine shoots in comparison with that of wild type CSS-01s, and RNAi transformants of LtLysM1 exhibited significantly decreased lesion length when compared with that of wild type CSS-01s. Moreover, LtLysM1 was confirmed to be a secreted protein by a yeast signal peptide trap assay. Transient expression in Nicotiana benthamiana together with protein immunoblotting confirmed that LtLysM1 was an N-glycosylated protein. In contrast to previously reported LysM protein Slp1 and OsCEBiP, LtLysM1 molecule did not interact with itself based on yeast two hybrid and co-immunoprecipitation assays. These results indicate that LtLysM1 is a secreted protein and functions as a critical virulence factor during the disease symptom development in woody plants.

Organize, Don't Agonize: Strategic Success of Phytophthora Species

Plants are constantly challenged by various environmental stressors ranging from abiotic-sunlight, elevated temperatures, drought, and nutrient deficits, to biotic factors-microbial pathogens and insect pests. These not only affect the quality of harvest but also the yield, leading to substantial annual crop losses, worldwide. Although plants have a multi-layered immune system, phytopathogens such as species of the oomycete genus Phytophthora, can employ elaborate mechanisms to breach this defense. For the last two decades, researchers have focused on the co-evolution between Phytophthora and interacting hosts to decouple the mechanisms governing their molecular associations. This has provided a comprehensive understanding of the pathobiology of plants affected by oomycetes. Ultimately, this is important for the development of strategies to sustainably improve agricultural production. Therefore, this paper discusses the present-day state of knowledge of the strategic mode of operation employed by species of Phytophthora for successful infection. Specifically, we consider motility, attachment, and host cell wall degradation used by these pathogenic species to obtain nutrients from their host. Also discussed is an array of effector types from apoplastic (hydrolytic proteins, protease inhibitors, elicitins) to cytoplastic (RxLRs, named after Arginine-any amino acid-Leucine-Arginine consensus sequence and CRNs, for CRinkling and Necrosis), which upon liberation can subvert the immune response and promote diseases in plants.

A Phytophthora capsici effector suppresses plant immunity via interaction with EDS1

EDS1 (Enhanced Disease Susceptibility 1) plays a crucial role in both effector-triggered immunity activation and plant basal defence. However, whether pathogen effectors can target EDS1 or an EDS1-related pathway to manipulate immunity is rarely reported. In this study, we identified a Phytophthora capsici Avirulence Homolog (Avh) RxLR (Arg-any amino acid-Leu-Arg) effector PcAvh103 that interacts with EDS1. We demonstrated that PcAvh103 can facilitate P. capsici infection and is required for pathogen virulence. Furthermore, genetic evidence showed that PcAvh103 contributes to virulence through targeting EDS1. Finally, PcAvh103 specifically interacts with the lipase domain of EDS1 and can promote the disassociation of EDS1-PAD4 (Phytoalexin Deficient 4) complex in planta. Together, our results revealed that the P. capsici RxLR effector PcAvh103 targets host EDS1 to suppress plant immunity, probably through disrupting the EDS1-PAD4 immune signalling pathway.

The PTI to ETI Continuum in Phytophthora-Plant Interactions

Phytophthora species are notorious pathogens of several economically important crop plants. Several general elicitors, commonly referred to as Pathogen-Associated Molecular Patterns (PAMPs), from Phytophthora spp. have been identified that are recognized by the plant receptors to trigger induced defense responses in a process termed PAMP-triggered Immunity (PTI). Adapted Phytophthora pathogens have evolved multiple strategies to evade PTI. They can either modify or suppress their elicitors to avoid recognition by host and modulate host defense responses by deploying hundreds of effectors, which suppress host defense and physiological processes by modulating components involved in calcium and MAPK signaling, alternative splicing, RNA interference, vesicle trafficking, cell-to-cell trafficking, proteolysis and phytohormone signaling pathways. In incompatible interactions, resistant host plants perceive effector-induced modulations through resistance proteins and activate downstream components of defense responses in a quicker and more robust manner called effector-triggered-immunity (ETI). When pathogens overcome PTI-usually through effectors in the absence of R proteins-effectors-triggered susceptibility (ETS) ensues. Qualitatively, many of the downstream defense responses overlap between PTI and ETI. In general, these multiple phases of Phytophthora-plant interactions follow the PTI-ETS-ETI paradigm, initially proposed in the zigzag model of plant immunity. However, based on several examples, in Phytophthora-plant interactions, boundaries between these phases are not distinct but are rather blended pointing to a PTI-ETI continuum.

Systematic Y2H Screening Reveals Extensive Effector-Complex Formation

During infection pathogens secrete small molecules, termed effectors, to manipulate and control the interaction with their specific hosts. Both the pathogen and the plant are under high selective pressure to rapidly adapt and co-evolve in what is usually referred to as molecular arms race. Components of the host's immune system form a network that processes information about molecules with a foreign origin and damage-associated signals, integrating them with developmental and abiotic cues to adapt the plant's responses. Both in the case of nucleotide-binding leucine-rich repeat receptors and leucine-rich repeat receptor kinases interaction networks have been extensively characterized. However, little is known on whether pathogenic effectors form complexes to overcome plant immunity and promote disease. Ustilago maydis, a biotrophic fungal pathogen that infects maize plants, produces effectors that target hubs in the immune network of the host cell. Here we assess the capability of U. maydis effector candidates to interact with each other, which may play a crucial role during the infection process. Using a systematic yeast-two-hybrid approach and based on a preliminary pooled screen, we selected 63 putative effectors for one-on-one matings with a library of nearly 300 effector candidates. We found that 126 of these effector candidates interacted either with themselves or other predicted effectors. Although the functional relevance of the observed interactions remains elusive, we propose that the observed abundance in complex formation between effectors adds an additional level of complexity to effector research and should be taken into consideration when studying effector evolution and function. Based on this fundamental finding, we suggest various scenarios which could evolutionarily drive the formation and stabilization of an effector interactome.

Effectors of Phytophthora pathogens are powerful weapons for manipulating host immunity

This article provides an overview of the interactions between Phytophthora effectors and plant immune system components, which form a cross-linked complex network that regulates plant pathogen resistance. Pathogens secrete numerous effector proteins into plants to promote infections. Several Phytophthora species (e.g., P. infestans, P. ramorum, P. sojae, P. capsici, P. cinnamomi, and P. parasitica) are notorious pathogens that are extremely damaging to susceptible plants. Analyses of genomic data revealed that Phytophthora species produce a large group of effector proteins, which are critical for pathogenesis. And, the targets and functions of many identified Phytophthora effectors have been investigated. Phytophthora effectors can affect various aspects of plant immune systems, including plant cell proteases, phytohormones, RNAs, the MAPK pathway, catalase, the ubiquitin proteasome pathway, the endoplasmic reticulum, NB-LRR proteins, and the cell membrane. Clarifying the effector-plant interactions is important for unravelling the functions of Phytophthora effectors during pathogenesis. In this article, we review the effectors identified in recent decades and provide an overview of the effector-directed regulatory network in plants following infections by Phytophthora species.

Plasmopara viticola effector PvRXLR131 suppresses plant immunity by targeting plant receptor-like kinase inhibitor BKI1

The grapevine downy mildew pathogen Plasmopara viticola secretes a set of RXLR effectors (PvRXLRs) to overcome host immunity and facilitate infection, but how these effectors function is unclear. Here, the biological function of PvRXLR131 was investigated via heterologous expression. Constitutive expression of PvRXLR131 in Colletotrichum gloeosporioides significantly enhanced its pathogenicity on grapevine leaves. Constitutive expression of PvRXLR131 in Arabidopsis promoted Pseudomonas syringae DC3000 and P. syringae DC3000 (hrcC^- ) growth as well as suppressed defence-related callose deposition. Transient expression of PvRXLR131 in Nicotiana benthamiana leaves could also suppress different elicitor-triggered cell death and inhibit plant resistance to Phytophthora capsici. Further analysis revealed that PvRXLR131 interacted with host Vitis vinifera BRI1 kinase inhibitor 1 (VvBKI1), and its homologues in N. benthamiana (NbBKI1) and Arabidopsis (AtBKI1). Moreover, bimolecular fluorescence complementation analysis revealed that PvRXLR131 interacted with VvBKI1 in the plasma membrane. Deletion assays showed that the C-terminus of PvRXLR131 was responsible for the interaction and mutation assays showed that phosphorylation of a conserved tyrosine residue in BKI1s disrupted the interaction. BKI1 was a receptor inhibitor of growth- and defence-related brassinosteroid (BR) and ERECTA (ER) signalling. When silencing of NbBKI1 in N. benthamiana, the virulence function of PvRXLR131 was eliminated, demonstrating that the effector activity is mediated by BKI1. Moreover, PvRXLR131-transgenic plants displayed BKI1-overexpression dwarf phenotypes and suppressed BR and ER signalling. These physiological and genetic data clearly demonstrate that BKI1 is a virulence target of PvRXLR131. We propose that P. viticola secretes PvRXLR131 to target BKI1 as a strategy for promoting infection.

A Phytophthora capsici Effector Targets ACD11 Binding Partners that Regulate ROS-Mediated Defense Response in Arabidopsis

Reactive oxygen species (ROS) play a vital role in plant immune response, but the genes involved in the regulation of ROS are scantily reported. Phytophthora pathogens produce a large number of effectors to promote infection, but the modes of action adopted are largely unknown. Here, we report that RxLR207 could activate ROS-mediated cell death in Nicotiana benthamiana and was essential for virulence of P. capsici. We found that this effector targeted BPA1 (binding partner of ACD11) and four members of BPLs (BPA1-Like proteins) in Arabidopsis, and the bpa1 and bpl mutants had enhanced ROS accumulation and cell death under biotic or abiotic stresses. Furthermore, we showed that BPA1 and several BPLs functioned redundantly in plant immunity to P. capsici. We discovered that BPA1 and all six BPLs interacted with ACD11, and stabilization of ACD11 was impaired in the bpa1, bpl2, bpl3, and bpl4 mutants. RxLR207 could promote the degradation of BPA1, BPL1, BPL2, and BPL4 to disrupt ACD11 stabilization in a 26S proteasome-dependent manner. Taken together, these findings indicate the important roles of Arabidopsis BPA1 and its homologs in ROS homeostasis and defense response, highlighting the usefulness of a pathogen effector-directed approach as a promising strategy for the discovery of novel plant immune regulators.

Arms race: diverse effector proteins with conserved motifs

Effector proteins play important roles in the infection by pathogenic oomycetes and fungi or the colonization by endophytic and mycorrhizal fungi. They are either translocated into the host plant cells via specific translocation mechanisms and function in the host's cytoplasm or nucleus, or they reside in the apoplast of the plant cells and act at the extracellular host-microbe interface. Many effector proteins possess conserved motifs (such as the RXLR, CRN, LysM, RGD, DELD, EAR, RYWT, Y/F/WXC or CFEM motifs) localized in their N- or C-terminal regions. Analysis of the functions of effector proteins, especially so-called "core effectors", is crucial for the understanding of pathogenicity/symbiosis mechanisms and plant defense strategies, and helps to develop breeding strategies for pathogen-resistant cultivars, and to increase crop yield and quality as well as abiotic stress resistance. This review summarizes current knowledge about these effector proteins with the conversed motifs and their involvement in pathogenic or mutualistic plant/fungal interactions.

Tomato Yellow Leaf Curl Virus V2 Interacts with Host Histone Deacetylase 6 To Suppress Methylation-Mediated Transcriptional Gene Silencing in Plants

Cytosine DNA methylation is a conserved epigenetic silencing mechanism that defends against biotic stresses such as geminivirus infection. As a countermeasure, geminiviruses encode proteins that inhibit methylation and transcriptional gene silencing (TGS). Previous studies showed that V2 protein of Tomato yellow leaf curl virus (TYLCV) functions as a TGS suppressor. However, how V2 mediates TGS suppression remains unknown. Here we show that V2 interacts directly with a Nicotiana benthamiana histone deacetylase 6 (NbHDA6), a homolog of Arabidopsis HDA6 (AtHDA6), known to be involved in gene silencing in cooperation with methyltransferase 1 (MET1). NbHDA6 genetically complemented a late-flowering phenotype and restored histone deacetylation of an AtHDA6 mutant. Furthermore, our investigation showed that NbHDA6 displayed histone deacetylase enzymatic activity, which was not inhibited by V2. Genetic analysis revealed that silencing of NbHDA6 expression resulted in enhanced susceptibility to TYLCV infection. In addition, methylation-sensitive PCR and bisulfite sequencing analysis showed that silencing of NbHDA6 expression caused reduced DNA methylation of the viral genome in infected plants. HDA6 was previously shown to recruit and physically interact with MET1 to function in gene silencing. Using competitive pulldown and coimmunoprecipitation assays, we demonstrated that V2 did not interact but competed with NbMET1 for direct binding to NbHDA6. These findings suggest that V2 interacts with host HDA6 and interferes with the recruitment of MET1 by HDA6, resulting in decreased methylation of the viral DNA genome by TGS with a concomitant increase in host susceptibility to TYLCV infection.IMPORTANCE Plants employ repressive viral genome methylation as an epigenetic defense against geminiviruses. In turn, geminiviruses encode proteins that inhibit methylation by TGS. Previous studies showed that TYLCV V2 can efficiently suppress TGS, but the mechanism remains unknown. We showed that V2 interacted with NbHDA6 but did not inhibit its enzymatic activity. As HDA6 is known to be involved in gene silencing in cooperation with MET1, we explored the relationship between V2, NbMET1, and NbHDA6. Our investigation showed that V2 did not interact but competed with NbMET1 for direct binding to NbHDA6. To our knowledge, this is the first report that viral proteins inhibit TGS by interacting with histone deacetylase but not by blocking the methyl cycle. This work provides an additional mechanism for TGS suppression by geminiviruses.

RiCRN1, a Crinkler Effector From the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis, Functions in Arbuscule Development

Arbuscular mycorrhizal (AM) symbiosis is one of the most prominent and beneficial plant–microbe interactions that facilitates mineral nutrition and confers tolerance to biotic and abiotic stresses. AM fungi colonize the root cortex and develop specialized structures called arbuscules where the nutrient exchange takes place. Arbuscule development is a highly controlled and coordinated process requiring the involvement of many plant proteins recruited at that interface. In contrast, much less is known about the fungal proteins involved in this process. Here, we have identified an AM fungal effector that participates in this developmental step of the symbiosis. RiCRN1 is a crinkler (CRN) effector that belongs to a subfamily of secreted CRN proteins from R. irregularis. CRNs have been so far only functionally characterized in pathogenic microbes and shown to participate in processes controlling plant cell death and immunity. RiCRN1 accumulates during symbiosis establishment parallel to MtPT4, the gene coding for an arbuscule-specific phosphate transporter. Expression in Nicotiana benthamiana leaves and in Medicago truncatula roots suggest that RiCRN1 is not involved in cell death processes. RiCRN1 dimerizes and localizes to nuclear bodies, suggesting that, similar to other CRNs, it functions in the plant nucleus. Downregulation of RiCRN1 using host-induced gene silencing led to an impairment of the symbiosis in M. truncatula and to a reduction of MtPT4, while ectopic expression of RiCRN1, surprisingly, led to a drastic reduction in arbuscule size that correlated with a decrease not only in MtPT4 but also in MtBCP1, a marker for initial stages of arbuscule development. Altogether, our results suggest that a tightly regulated expression in time and space of RiCRN1 is critical for symbiosis progression and for the proper initiation of arbuscule development.

Random mutagenesis screen shows that Phytophthora capsici CRN83_152-mediated cell death is not required for its virulence function(s)

With the increasing availability of plant pathogen genomes, secreted proteins that aid infection (effectors) have emerged as key factors that help to govern plant-microbe interactions. The conserved CRN (CRinkling and Necrosis) effector family was first described in oomycetes by their capacity to induce host cell death. Despite recent advances towards the elucidation of CRN virulence functions, the relevance of CRN-induced cell death remains unclear. In planta over-expression of PcCRN83_152, a CRN effector from Phytophthora capsici, causes host cell death and boosts P. capsici virulence. We used these features to ask whether PcCRN83_152-induced cell death is linked to its virulence function. By randomly mutating this effector, we generated PcCRN83_152 variants with no cell death (NCD) phenotypes, which were subsequently tested for activity towards enhanced virulence. We showed that a subset of PcCRN83_152 NCD variants retained their ability to boost P. capsici virulence. Moreover, NCD variants were shown to have a suppressive effect on PcCRN83_152-mediated cell death. Our work shows that PcCRN83_152-induced cell death and virulence function can be separated. Moreover, if these findings hold true for other cell death-inducing CRN effectors, this work, in turn, will provide a framework for studies aimed at unveiling the virulence functions of these effectors.

Ralstonia solanacearum novel E3 ubiquitin ligase (NEL) effectors RipAW and RipAR suppress pattern-triggered immunity in plants

Ralstonia solanacearum is the causal agent of bacterial wilt in solanaceous crops. This pathogen injects more than 70 effector proteins into host plant cells via the Hrp type III secretion system to cause a successful infection. However, the function of these effectors in plant cells, especially in the suppression of plant immunity, remains largely unknown. In this study, we characterized two Ralstonia solanacearum effectors, RipAW and RipAR, which share homology with the IpaH family of effectors from animal and plant pathogenic bacteria, that have a novel E3 ubiquitin ligase (NEL) domain. Recombinant RipAW and RipAR show E3 ubiquitin ligase activity in vitro. RipAW and RipAR localized to the cytoplasm of plant cells and significantly suppressed pattern-triggered immunity (PTI) responses such as the production of reactive oxygen species and the expression of defence-related genes when expressed in leaves of Nicotiana benthamiana. Mutation in the conserved cysteine residue in the NEL domain of RipAW completely abolished the E3 ubiquitin ligase activity in vitro and the ability to suppress PTI responses in plant leaves. These results indicate that RipAW suppresses plant PTI responses through the E3 ubiquitin ligase activity. Unlike other members of the IpaH family of effectors, RipAW and RipAR had no leucine-rich repeat motifs in their amino acid sequences. A conserved C-terminal region of RipAW is indispensable for PTI suppression. Transgenic Arabidopsis plants expressing RipAW and RipAR showed increased disease susceptibility, suggesting that RipAW and RipAR contribute to bacterial virulence in plants.

A Perspective on CRN Proteins in the Genomics Age: Evolution, Classification, Delivery and Function Revisited

Plant associated microbes rely on secreted virulence factors (effectors) to modulate host immunity and ensure progressive infection. Amongst the secreted protein repertoires defined and studied in pathogens to date, the CRNs (for CRinkling and Necrosis) have emerged as one of only a few highly conserved protein families, spread across several kingdoms. CRN proteins were first identified in plant pathogenic oomycetes where they were found to be modular factors that are secreted and translocated inside host cells by means of a conserved N-terminal domain. Subsequent localization and functional studies have led to the view that CRN C-termini execute their presumed effector function in the host nucleus, targeting processes required for immunity. These findings have led to great interest in this large protein family and driven the identification of additional CRN-like proteins in other organisms. The identification of CRN proteins and subsequent functional studies have markedly increased the number of candidate CRN protein sequences, expanded the range of phenotypes tentatively associated with function and revealed some of their molecular functions toward virulence. The increased number of characterized CRNs also has presented a set of challenges that may impede significant progress in the future. Here, we summarize our current understanding of the CRNs and re-assess some basic assumptions regarding this protein family. We will discuss the latest findings on CRN biology and highlight exciting new hypotheses that have emanated from the field. Finally, we will discuss new approaches to study CRN functions that would lead to a better understanding of CRN effector biology as well as the processes that lead to host susceptibility and immunity.

RXLR and CRN Effectors from the Sunflower Downy Mildew Pathogen Plasmopara halstedii Induce Hypersensitive-Like Responses in Resistant Sunflower Lines

Plasmopara halstedii is an obligate biotrophic oomycete causing downy mildew disease on sunflower, Helianthus annuus, an economically important oil crop. Severe symptoms of the disease (e.g., plant dwarfism, leaf bleaching, sporulation and production of infertile flower) strongly impair seed yield. Pl resistance genes conferring resistance to specific P. halstedii pathotypes were located on sunflower genetic map but yet not cloned. They are present in cultivated lines to protect them against downy mildew disease. Among the 16 different P. halstedii pathotypes recorded in France, pathotype 710 is frequently found, and therefore continuously controlled in sunflower by different Pl genes. High-throughput sequencing of cDNA from P. halstedii led us to identify potential effectors with the characteristic RXLR or CRN motifs described in other oomycetes. Expression of six P. halstedii putative effectors, five RXLR and one CRN, was analyzed by qRT-PCR in pathogen spores and in the pathogen infecting sunflower leaves and selected for functional analyses. We developed a new method for transient expression in sunflower plant leaves and showed for the first time subcellular localization of P. halstedii effectors fused to a fluorescent protein in sunflower leaf cells. Overexpression of the CRN and of 3 RXLR effectors induced hypersensitive-like cell death reactions in some sunflower near-isogenic lines resistant to pathotype 710 and not in susceptible corresponding lines, suggesting they could be involved in Pl loci-mediated resistances.